Multiple Choice
A solution with a hydroxide ion concentration of 4.15 × 10^-7 M is considered a weak base. What is the hydrogen ion concentration of this solution?
17. Acid and Base Equilibrium
pH of Weak Bases
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Calculate the pH of a 0.10 M solution of hydrazine, N2H4, given that the Kb for hydrazine is 1.3×10⁻⁶.
A
pH = 3.87
B
pH = 7.00
C
pH = 11.00
D
pH = 10.13
Verified step by step guidance1
Identify that hydrazine (N2H4) is a weak base and will partially ionize in water. The ionization can be represented by the equation: N2H4 + H2O ⇌ N2H5⁺ + OH⁻.
Write the expression for the base dissociation constant (Kb) for hydrazine: Kb = [N2H5⁺][OH⁻] / [N2H4].
Set up an ICE (Initial, Change, Equilibrium) table to determine the concentrations of the species at equilibrium. Initially, [N2H4] = 0.10 M, and [N2H5⁺] = [OH⁻] = 0 M. Let x be the change in concentration of N2H5⁺ and OH⁻ at equilibrium.
Substitute the equilibrium concentrations into the Kb expression: Kb = (x)(x) / (0.10 - x). Since Kb is small, assume x << 0.10, simplifying the expression to Kb ≈ x² / 0.10.
Solve for x, which represents [OH⁻], and then calculate the pOH using the formula pOH = -log[OH⁻]. Finally, convert pOH to pH using the relationship pH + pOH = 14.
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